Radio frequency identification (RFID) systems typically include at least one host reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. The transponder may be an active or passive radio frequency communication device which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder may be embedded in a portable substrate, such as a card, tag, or the like, carried by a person or an article to be identified or otherwise characterized by the reader. A passive transponder is characterized as being dependent on the host reader for its power source. The host reader “excites” or powers up the passive transponder by transmitting excitation signals into the space surrounding the reader, which are received by the transponder and provide the operating power for the circuitry of the recipient transponder. In contrast, an active transponder is powered up by its own internal power source, such as a battery, which provides the operating power for the transponder circuitry.
Once the active or passive transponder is powered up, the transponder communicates information, such as identity data or other characterizing data stored in the memory of the transponder, to the reader and the reader can likewise communicate information back to the transponder without the reader and transponder coming into contact with one another. The transponder transmits transponder data signals in the form of electromagnetic waves via a transponder antenna into the surrounding space occupied by the reader. The reader receives the transponder data signals on a reader antenna and the reader contains its own circuitry to “read” the transponder data signals, i.e., extract the data from the transponder data signals. Reading a transponder data signal requires the reader circuitry to process a transponder data signal in a manner which typically comprises conditioning the transponder data signal by means including an amplifier. The resulting conditioned signal is then demodulated to extract the transponder data therefrom.
RFID systems are generally characterized by a number of parameters relating to transmission and processing of the data signals from either the transponder or the reader. Such parameters include the carrier frequency of the data signals, the transfer rate of the data in the data signals, and the type of modulation of the data signals. In particular, data signals communicated between the transponder and reader of a given RFID system are usually at a specified standard carrier frequency, which is characteristic of the given RFID system. For example, RFID systems, which employ transponders of the type conventionally termed proximity cards or proximity tags, typically communicate by means of data signals at a carrier frequency within a range of 100 to 150 kHz. This carrier frequency range is nominally referred to herein as 125 kHz carrier frequency and is deemed a low frequency. In contrast, RFID systems, which employ transponders of the type conventionally termed smart cards, typically communicate by means of data signals at a higher frequency of about 13.56 MHz.
The transfer rate of digital data communicated between the transponder and reader of a given RFID system via the data signals is commonly at one of a number of specified standard data rates, which is also characteristic of the given RFID system. The specified data rates are usually a function of the carrier frequency for the given RFID system. For example, RFID systems operating at the 125 kHz carrier frequency typically employ a relatively low data rate on the order of a few kilobits per second. For RFID systems operating at the 13.56 MHz carrier frequency, one particular industry standard specifies a low data rate of about 6 kilobits per second and a high data rate of about 26 kilobits per second. Another industry standard specifies an even higher data rate of 106 kilobits per second for RFID systems operating at the 13.56 MHz carrier frequency.
The type of modulation applied to data signals in a given RFID system is also characteristic of the given RFID system. Among the different modulation types available to RFID systems are frequency shift keying (FSK), phase shift keying (PSK) and amplitude shift keying (ASK).
Referring to FIG. 1, a representative transponder and reader of an RFID system, which are designated 12 and 14, respectively, are shown positioned relative to one another along a linear x-axis representing distance. For purposes of illustration, the read range of a reader has been described above in the context of a one-dimensional linear model. However, in practice it is understood that the read range of a reader is a three-dimensional space to which the above-recited one-dimensional model is readily applicable. In any case, communication between the transponder 12 and reader 14 is only enabled when the transponder 12 and reader 14 are sufficiently close to one another that transponder data signals received by the reader 14 are of sufficient strength that the reader 14 is able to demodulate the transponder data signals and extract the data therefrom. When the reader 14 is fixed at a position x=0, the furthest point on the x-axis where the transponder 12 can be positioned while still enabling communication between the reader 14 and transponder 12 is designated x=M and is termed the read range maximum of the reader 14. As such, the entire read range of the reader 14 is designated 0≦x≦M.
It has been found that as the transponder 12 moves from the read range maximum M to points within the read range closer to the reader 14 designated H1<x<M, which are collectively termed the far read range segment, the amplitude of the transponder data signals received by the reader 14 generally increases, thereby causing the reader amplifier to begin clipping the received transponder data signal. Because different portions of the passband of the reader antenna have more gain than others, some frequencies of the received transponder data signal are clipped sooner than others. At some point within the far read range segment designated x=H1 and termed the near end of the far read range segment, some, but not all, of the received transponder data signal is clipped by the reader amplifier to the extent that the reader 14 is unable to properly demodulate the conditioned transponder data signal from the reader amplifier because the reader amplifier has overly distorted the transponder data signal. Accordingly, when the transponder 12 reaches the near end of the far read range segment H1, the reader 14 operating at its normal settings is unable to read the transponder data signal.
As the transponder 12 continues to approach the reader 14 from the near end of the far read range segment H1, the reader amplifier clips even more of the received transponder data signal. Nevertheless, at some point designated x=H2 and termed the far end of the near read range segment, the distortion of the transponder data signal diminishes to the extent that the reader 14 is again able to demodulate the conditioned output signal of the reader amplifier. Accordingly, the segment of the read range designated H2≦x≦H1, wherein H1<M and H2>0, is termed a “hole” and is characterized as a segment of the read range closer to the reader 14 than the far read range segment where the reader 14 is unable to read the transponder data signal. The segment of the read range designated 0≦x<H1 is termed the near read range segment and is characterized as a segment of the read range separated from the far read range segment by a hole where the reader 14 is once again able to read the transponder data signal.
The present invention recognizes a need for a reader which is capable of reading received transponder data signals across essentially the entirety of the read range of the reader. Accordingly, it is an object of the present invention to enhance the signal reading capability of a reader when a transponder is positioned within a hole in the read range of the reader. More particularly, it is an object of the present invention to provide a reader which effectively compensates for holes in the read range of the reader. Still more particularly, it is an object of the present invention to provide a reader which is configured to selectively adjust the gain of the reader amplifier to compensate for holes in the read range of the reader. These objects and others are accomplished in accordance with the invention described hereafter.